COORDINATION GEOMETRY OF CADMIUM AT THE ZINC AND COPPER SITES OF SUPEROXIDE DISMUTASES - A STUDY USING PERTURBED ANGULAR-CORRELATION OF GAMMA-RAYS FROM EXCITED CD-111

Cd-111 time-differential perturbed gamma-gamma angular correlation (PAC) has been used to investigate the Zn site in yeast and bovine copper and zinc-containing superoxide dismutases by substitution of the zinc ions with excited Cd-111(2+) ions. The PAC spectra obtained from the enzymes in aqueous solution reveal a single coordination geometry of Cd-1112+, showing that the coordination of Cd-1112+ to the Zn site in the two subunits is identical. Furthermore, the PAC spectra of the yeast and bovine enzymes show that the Zn sites are very similar in the two enzymes. The PAC experiments show a clear difference depending on whether the copper ion is in the oxidized or the reduced state. In the latter case the results resemble those obtained for derivatives with no metal ion at the Cu site. Hence, the coordination geometry of the Zn site in these two situations must be similar, and it is very unlikely that the imidazole ring of His61 bridges the two metal ions in the reduced enzyme. The PAC spectrum of Cd-1112+ ions at the Zn site with copper (II) ions at the Cu site is in agreement with that predicted by applying the angular overlap model (AOM) to the known crystal structure of the bovine enzyme, with known nuclear quadrupole interactions for the ligands involved. Furthermore, results from experiments with copper in the reduced state show that reduction of the copper ion causes a significant change at the Zn site. An explanation for this conformational change has been proposed by computer modelling. The PAC experiments also show that it is possible to incorporate cadmium ions into the Cu site in the absence of copper ions, and the result has also been interpreted in terms of the AOM.